Image projection system with a polarizing beam splitter

ABSTRACT

An image projection system has a wire grid polarizing beam splitter which functions as both the polarizer and the analyzer in the system. A light source produces a source light beam directed at the beam splitter which reflects one polarization and transmits the other. A liquid crystal array is disposed in either the reflected or transmitted beam. The array modulates the polarization of the beam, encoding image information thereon, and directs the modulated beam back to the beam splitter. The beam splitter again reflects one polarization and transmits the other so that the encoded image is either reflected or transmitted to a screen. The beam splitter can be an embedded wire grid polarizer with an array of parallel, elongated, spaced-apart elements sandwiched between first and second layers. The elements form a plurality of gaps between the elements which provide a refractive index less than the refractive index of the first or second layers.

This application is a continuation-in-part of U.S. Ser. No. 09/363,256filed Jul. 28, 1999, now U.S. Pat. No. 6,234,634.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image projection system operablewithin the visible spectrum which includes a polarizing beam splitterwhich reflects one linear polarization of light and transmits the other.More particularly, the present invention relates to such an imageprojection system with a beam splitter that is comprised of a pluralityof elongated, reflective elements which are disposed on a substrate insuch a way to reduce geometric distortions, astigmatism and/or coma inthe resulting light beam, and/or which are embedded or otherwiseconfigured to protect the elements.

2 . Related Art

Polarized light is necessary in certain applications, such as projectionliquid crystal displays (LCD). Such a display is typically comprised ofa light source; optical elements, such as lenses to gather and focus thelight; a polarizer that transmits one polarization of the light to theliquid crystal array; a liquid crystal array for manipulating thepolarization of the light to encode image information thereon; means foraddressing each pixel of the array to either change or retain thepolarization; a second polarizer (called an analyzer) to reject theunwanted light from the selected pixels; and a screen upon which theimage is focused.

It is possible to use a single polarizing beam splitter (PBS) to serveboth as the first polarizer and the second polarizer (analyzer). If theliquid crystal array is reflective, for example a Liquid Crystal OnSilicon (LCOS) light valve, it can reflect the beam that comes from thepolarizer directly back to the polarizer after encoding the image bymodifying the polarization of selected pixels. Such a system wasenvisioned by Takanashi (U.S. Pat. No. 5,239,322). The concept waselaborated by Fritz and Gold (U.S. Pat. No. 5,513,023). These similarapproaches would provide important advantages in optical layout andperformance. Neither, however, has been realized in practice because ofdeficiencies in conventional polarizing beam splitters. Thedisadvantages of using conventional polarizing beam splitters inprojection liquid crystal displays includes images that are not bright,have poor contrast, and have non-uniform color balance or non-uniformintensity (due to non-uniform performance over the light cone). Inaddition, many conventional polarizing beam splitters are short-livedbecause of excessive heating, and are very expensive.

In order for such an image projection system to be commerciallysuccessful, it must deliver images which are significantly better thanthe images provided by conventional cathode ray tube (CRT) televisiondisplays because it is likely that such a system will be more expensivethan conventional CRT technology. Therefore, the image projection systemmust provide (1) bright images with the appropriate colors or colorbalance; (2) have good image contrast; and (3) be as inexpensive aspossible. An improved polarizing beam splitter (PBS) is an importantpart of achieving this goal because the PBS is a limiting componentwhich determines the potential performance of the display system.

The PBS characteristics which significantly affect the displayperformance are (1) the angular aperture, or the f-number, at which thepolarizer can function; (2) the absorption, or energy losses, associatedwith the use of the PBS; and (3) the durability of the PBS. In optics,the angular aperture or f-number describes the angle of the light conewhich the PBS can use and maintain the desired performance level. Largercones, or smaller f-numbers, are desired because the larger cones allowfor more light to be gathered from the light source, which leads togreater energy efficiency and more compact systems.

The absorption and energy losses associated with the use of the PBSobviously affect the brightness of the system since the more light lostin the optics, the less light remains which can be projected to the viewscreen. In addition, the amount of light energy which is absorbed by thepolarizer will affect its durability, especially in such imageprojection systems in which the light passing through the optical systemis very intense, on the order of watts per square centimeter. Light thisintense can easily damage common polarizers, such as Polaroid sheets. Infact, the issue of durability limits the polarizers which can be used inthese applications.

Durability is also important because the smaller and lighter theprojection system can be made, the less expensive and more desirable isthe product. To accomplish this goal, however, the light intensity mustbe raised even higher, further stressing the PBS, and shortening itsuseful life.

A problematic disadvantage of conventional PBS devices is poorconversion efficiency, which is the primary critical performance factorin displays. Conversion efficiency is a measure describing how much ofthe electrical power required by the light source is translated intolight intensity power on the screen or panel that is observed by peopleviewing it. It is expressed as the ratio of total light power on thescreen divided by the electrical power required by the light source. Theconventional units are lumens per watt. A high ratio is desirable for anumber of reasons. For example, a low conversion efficiency will requirea brighter light source, with its accompanying larger power supply,excess heat, larger enclosures and cabinet, etc. In addition, all ofthese consequences of low conversion efficiency raise the cost of theprojection system.

A fundamental cause of low conversion efficiency is poor opticalefficiency, which is directly related to the f-number of the opticalsystem. A system which has an f-number which is half the f-number of anotherwise equivalent system has the potential to be four times asefficient in gathering light from the light source. Therefore, it isdesirable to provide an improved polarizing beam splitter (PBS) whichallows more efficient harvesting of light energy by offering asignificantly smaller potential f-number (larger angular aperture), andtherefore increases the conversion efficiency, as measured inlumens/watt.

There are several reasons for the poor performance of conventionalpolarizing beam splitters with respect to conversion efficiency whenthey are used as beam splitters in projection systems. First, currentbeam splitters work poorly if the light does not strike them at acertain angle (or at least, within a narrow cone of angles about thisprincipal angle of incidence). Deviation of the principal ray from thisangle causes each type of polarizing beam splitter to degrade theintensity, the purity of polarization, and/or the color balance. Thisapplies to the beam coming from the light source as well as to the beamreflected from the liquid crystal array. This principal angle dependsupon the design and construction of the PBS as well as the physics ofthe polarization mechanism employed in these various beam splitters.Currently available polarizing beam splitters are not capable ofoperating efficiently at angles far from their principal polarizingangles in the visible portion of the electromagnetic spectrum. Thisrestriction makes it impossible to implement certain promising opticallayouts and commercially promising display designs.

Even if the principal ray strikes the polarizer at the best angle forseparating the two polarizations, the other rays cannot diverge far fromthis angle or their visual qualities will be degraded. This is a seriousdeficiency in a display apparatus because the light striking thepolarizer must be strongly convergent or divergent to make efficient useof the light emitted by typical light sources. This is usually expressedas the f-number of the optical system. For a single lens, the f-numberis the ratio of the aperture to the focal length. For optical elementsin general, the F-number is defined as

F/#=1/(2 n sin Θ)

where n is the refractive index of the space within which the opticalelement is located, and Θ is the half cone angle. The smaller theF-number, the greater the radiant flux, Φ_(c), collected by the lens,and the more efficient the device will be for displaying a bright image.The radiant flux increases as the inverse square of the F/#. In anoptical train, the optical element with the largest F/# will be thelimiting factor in its optical efficiency. For displays usingtraditional polarizers, the limiting element is nearly always thepolarizer, and thus the PBS limits the conversion efficiency. It wouldclearly be beneficial to develop a type of PBS that has a smaller F/#than any that are currently available.

Because traditional polarizers with small F/#s have not been available,designers typically have addressed the issue of conversion efficiency byspecifying a smaller, brighter light source. Such sources, typically arclamps, are available, but they require expensive power supplies that areheavy, bulky, and need constant cooling while in operation. Cooling fanscause unwanted noise and vibration. These features are detrimental tothe utility of projectors and similar displays. Again, a PBS with asmall F/# would enable efficient gathering of light from low-power,quiet, conventional light sources.

Another key disadvantage of conventional polarizing beam splitters is alow extinction, which results in poor contrast in the image. Extinctionis the ratio of the light transmitted through the polarizer of thedesired polarization to the light rejected of the undesiredpolarization. In an efficient display, this ratio must be maintained ata minimum value over the entire cone of light passing through the PBS.Therefore, it is desirable to provide a polarizing beam splitter whichhas a high extinction ratio resulting in a high contrast image.

A third disadvantage of conventional polarizing beam splitters is anon-uniform response over the visible spectrum, or poor color fidelity.The result is poor color balance which leads to further inefficiency inthe projection display system as some light from the bright colors mustbe thrown away to accommodate the weaknesses in the polarizing beamsplitter. Therefore, it is desirable to provide an improved polarizingbeam splitter that has a uniform response over the visible spectrum, (orgood color fidelity) giving an image with good color balance with betterefficiency. The beam splitter must be achromatic rather than distort theprojected color, and it must not allow crosstalk between thepolarizations because this degrades image acuity and contrast. Thesecharacteristics must apply over all portions of the polarizer and overall angles of light incidence occurring at the polarizer. The termspathic has been coined (R. C. Jones, Jour. Optical Soc. Amer. 39, 1058,1949) to describe a polarizer that conserves cross-sectional area, solidangle, and the relative intensity distribution of wavelengths in thepolarized beam. A PBS that serves as both a polarizer and analyzer mustbe spathic for both transmission and reflection, even in light beams oflarge angular aperture.

A fourth disadvantage of conventional polarizing beam splitters is poordurability. Many conventional polarizing beam splitters are subject todeterioration caused by excessive heating and photochemical reactions.Therefore, it is desirable to provide an improved polarizing beamsplitter that can withstand an intense photon flux for thousands ofhours without showing signs of deterioration. In addition, it isdesirable to provide a polarizing beam splitter that is amenable toeconomical, large scale fabrication.

The need to meet these, and other, criteria has resulted in only a fewtypes of polarizers finding actual use in a projection system. Manyattempts have been made to incorporate both wide angular aperture andhigh fidelity polarization into the same beam splitting device. Therelative success of these efforts is described below. Thin filminterference filters are the type of polarizer cited most frequently inefforts to make a polarizing beam splitter that is also used as ananalyzer. MacNeille was the first to describe such a polarizer that waseffective over a wide spectral range (U.S. Pat. No. 2,403,731). It iscomposed of thin-film multi-layers set diagonally to the incident light,typically within a glass cube, so it is bulky and heavy compared to asheet polarizer. What is more, it must be designed for a single angle ofincidence, usually 45°, and its performance is poor if light is incidentat angles different from this by even as little as 2°. Others haveimproved on the design (e.g. J. Mouchart, J. Begel, and E. Duda, AppliedOptics 28, 2847-2853, 1989; and L. Li and J. A. Dobrowolski, AppliedOptics 13, 2221-2225, 1996). All of them found it necessary to seriouslyreduce the wavelength range if the angular aperture is to be increased.This can be done in certain designs (U.S. Pat. Nos. 5,658,060 and5,798,819) in which the optical design divides the light intoappropriate color bands before it arrives at the polarizing beamsplitter. In this way, it is possible to reduce the spectral bandwidthdemands on the beam splitter and expand its angular aperture, but theadditional components and complexity add significant cost, bulk, andweight to the system.

Even so, these improved beam splitter cubes are appearing on the market,and are currently available from well known vendors such as Balzers andOCLI. They typically offer an F/# of f/2.5-f/2.8, which is a significantimprovement over what was available 2 years ago, but is still far fromthe range of F/1.2-F/2.0 which is certainly within reach of the otherkey components in optical projection systems. Reaching these f-numbershas the potential to improve system efficiency by as much as a factor of4. They would also enable the projection display engineer to makepreviously impossible design trade-offs to achieve other goals, such asreduced physical size and weight, lower cost, etc.

In a technology far from visible optics, namely radar, wire grids havebeen used successfully to polarize long wavelength radar waves. Thesewire grid polarizers have also been used as reflectors. They are alsowell known as optical components in the infrared (IR), where they areused principally as transmissive polarizer elements.

Although it has not been demonstrated, some have postulated possible useof a wire grid polarizer in display applications in the visible portionof the spectrum. For example, Grinberg (U.S. Pat. No. 4,688,897)suggested that a wire grid polarizer serve as both a reflector and anelectrode (but not simultaneously as an analyzer) for a liquid crystaldisplay.

Others have posed the possible use of a wire grid polarizer in place ofa dichroic polarizer to improve the efficiency of virtual image displays(see U.S. Pat. No. 5,383,053). The need for contrast or extinction inthe grid polarizer, however, is explicitly dismissed, and the grid isbasically used as a polarization sensitive beam steering device. It doesnot serve the purpose of either an analyzer, or a polarizer, in the U.S.Pat. No. 5,383,053 patent. It is also clear from the text that abroadband polarizing cube beam splitter would have served the purpose aswell, if one had been available. This technology, however, is dismissedas being too restricted in acceptance angle to even be functional, aswell as prohibitively expensive.

Another patent (U.S. Pat. No. 4,679,910) describes the use of a gridpolarizer in an imaging system designed for the testing of IR camerasand other IR instruments. In this case, the application requires a beamsplitter for the long wavelength infra-red, in which case a gridpolarizer is the only practical solution. The patent does not suggestutility for the visible range or even mention the need for a largeangular aperture. Neither does it address the need for efficientconversion of light into a viewable image, nor the need for broadbandperformance.

Other patents also exist for wire-grid polarizers in the infraredportion of the spectrum (U.S. Pat. Nos. 4,514,479, 4,743,093; and5,177,635, for example). Except for the exceptions just cited, theemphasis is solely on the transmission performance of the polarizer inthe IR spectrum.

These references demonstrate that it has been known for many years thatwire-grid arrays can function generally as polarizers. Nevertheless,they apparently have not been proposed and developed for imageprojection systems. One possible reason that wire grid polarizers havenot been applied in the visible spectrum is the difficulty ofmanufacture. U.S. Pat. No. 4,514,479 teaches a method of holographicexposure of photoresist and subsequent etching in an ion mill to make awire grid polarizer for the near infrared region; in U.S. Pat. No.5,122,907, small, elongated ellipsoids of metal are embedded in atransparent matrix that is subsequently stretched to align their longaxes of the metal ellipsoids to some degree. Although the transmittedbeam is polarized, the device does not reflect well. Furthermore, theellipsoid particles have not been made small enough to be useful in thevisible part of the electromagnetic spectrum. Accordingly, practicalapplications have been generally limited to the longer wavelengths ofthe IR spectrum.

Another prior art polarizer achieves much finer lines by grazing angleevaporative deposition (U.S. Pat. No. 4,456,515). Unfortunately, thelines have such small cross sections that the interaction with thevisible light is weak, and so the optical efficiency is too poor for usein the production of images. As in several of these prior art efforts,this device has wires with shapes and spacings that are largely random.Such randomness degrades performance because regions of closely spacedelements do not transmit well, and regions of widely spaced elementshave poor reflectance. The resulting degree of polarization (extinction)is less than maximal if either or both of these effects occur, as theysurely must if placement has some randomness to it.

For perfect (and near perfect) regularity, the mathematics developed forgratings apply well. Conversely, for random wires (even if they all havethe same orientation) the theory of scattering provides the bestdescription. Scattering from a single cylindrical wire has beendescribed (H. C. Van de Hulst, Light Scattering by Small Particles,Dover, 1981). The current random-wire grids have wires embeddedthroughout the substrate. Not only are the positions of the wiressomewhat random, but the diameters are as well. It is clear that thephases of the scattered rays will be random, so the reflection will notbe strictly specular and the transmission will not retain high spacialor image fidelity. Such degradation of the light beam would prevent itsuse for transfer of well resolved, high-information density images.

Nothing in the prior art indicates or suggests that an ordered array ofwires can or should be made to operate over the entire visible range asa spathic PBS, at least at the angles required when it serves both as apolarizer and analyzer. Indeed, the difficulty of making the narrow,tall, evenly spaced wires that are required for such operation has beengenerously noted (see Zeitner, et. al. Applied Optics, 38, 11 pp.2177-2181 (1999), and Schnabel, et. al., Optical Engineering 38,2 pp.220-226 (1999)). Therefore, it is not surprising that the prior art forimage projection similarly makes no suggestion for use of a spathic PBSas part of a display device.

Tamada and Matsumoto (U.S. Pat. No. 5,748,368) disclose a wire gridpolarizer that operates in both the infrared and a portion of thevisible spectrum; however, it is based on the concept that large, widelyspaced wires will create resonance and polarization at an unexpectedlyshort wavelength in the visible. Unfortunately, this device works wellonly over a narrow band of visible wavelengths, and not over the entirevisible spectrum. It is therefore not suitable for use in producingimages in full color. Accordingly, such a device is not practical forimage display because a polarizer must be substantially achromatic foran image projection system.

Another reason wire grid polarizers have been overlooked is the commonand long standing belief that the performance of a typical wire gridpolarizer becomes degraded as the light beam's angle of incidencebecomes large (G. R. Bird and M. Parrish, Jr., “The Wire Grid as aNear-Infrared Polarizer,” J. Opt. Soc. Am., 50, pp. 886-891, (1960); theHandbook of Optics, Michael Bass, Volume II, p. 3-34, McGraw-Hill(1995)). There are no reports of designs that operate well for anglesbeyond 35° incidence in the visible portion of the spectrum. Nor hasanyone identified the important design factors that cause thislimitation of incidence angle. This perceived design limitation becomeseven greater when one realizes that a successful beam splitter willrequire suitable performance in both transmission and reflectionsimultaneously.

This important point deserves emphasis. The extant literature and patenthistory for wire grid polarizers in the IR and the visible spectra hasalmost entirely focused on their use as transmission polarizers, and noton reflective properties. Wire grid polarizers have been attempted andreported in the technical literature for decades, and have becomeincreasingly common since the 1960s. Despite the extensive work done inthis field, there is very little, if any, detailed discussion of theproduction and use of wire grid polarizers as reflective polarizers, andnothing in the literature concerning their use as both transmissive andreflective polarizers simultaneously, as would be necessary in a spathicpolarizing beam splitter for use in imaging devices. From the lack ofdiscussion in the literature, a reasonable investigator would concludethat any potential use of wire grid polarizers as broadband visible beamsplitters is not apparent, or that it was commonly understood by thetechnical community that their use in such an application was notpractical.

Because the conventional polarizers described above were the only onesavailable, it was impossible for Takanashi (U.S. Pat. No. 5,239,322) toreduce his projection device to practice with anything but the mostmeager results. No polarizer was available which supplied theperformance required for the Takanashi invention, namely, achromaticityover the visible part of the spectrum, wide angular acceptance, lowlosses in transmission and reflection of the desired lightpolarizations, and good extinction ratio.

There are several important features of an image display system whichrequire specialized performance of transmission and reflectionproperties. For a projector, the product of p-polarization transmissionand s-polarization reflection (R_(s)T_(p)) must be large if source lightis to be efficiently placed on the screen. On the other hand, for theresolution and contrast needed to achieve high information density onthe screen, it is important that the converse product (R_(p)T_(s)) bevery small (i.e. the transmission of s-polarized light multiplied by thereflection of p-polarized light must be small).

Another important feature is a wide acceptance angle. The acceptanceangle must be large if light gathering from the source, and hence theconversion efficiency, is maximized. It is desirable that cones of light(either diverging or converging) with half-angles greater than 20° beaccepted.

An important consequence of the ability to accept larger light cones andwork well at large angles is that the optical design of the imagingsystem is no longer restricted. Conventional light sources can be thenbe used, bringing their advantages of low cost, cool operation, smallsize, and low weight. A wide range of angles makes it possible for thedesigner to position the other optical elements in favorable positionsto improve the size and operation of the display.

Another important feature is size and weight. The conventionaltechnology requires the use of a glass cube. This cube imposes certainrequirements and penalties on the system. The requirements imposedinclude the need to deal with thermal loading of this large piece ofglass and the need for high quality materials without stressbirefringence, etc., which impose additional cost. In addition, theextra weight and bulk of the cube itself poses difficulties. Thus, it isdesirable that the beam splitter not occupy much volume and does notweigh very much.

Another important feature is robustness. Modern light sources generatevery high thermal gradients in the polarizer immediately after the lightis switched on. At best, this can induce thermal birefringence whichcauses cross talk between polarizations. What is more, the long durationof exposure to intense light causes some materials to change properties(typically yellowing from photo-oxidation). Thus, it is desirable forthe beam splitter to withstand high temperatures as well as long periodsof intense radiation from light sources.

Still another important feature is uniform extinction (or contrast)performance of the beam splitter over the incident cone of light. AMcNeille-type thin film stack polarizer produces polarized light due tothe difference in reflectivity of S-polarized light as opposed toP-polarized light. Since the definition of S and P polarization dependson the plane of incidence of the light ray, which changes orientationwithin the cone of light incident on the polarizer, a McNeille-typepolarizer does not work equally well over the entire cone. This weaknessin McNeille-type polarizers is well known. It must be addressed inprojection system design by restricting the angular size of the cone oflight, and by compensation elsewhere in the optical system through theuse of additional optical components. This fundamental weakness ofMcNeille prisms raises the costs and complexities of current projectionsystems, and limits system performance through restrictions on thef-number or optical efficiency of the beam splitter.

Other important features include ease of alignment. Production costs andmaintenance are both directly affected by assembly criteria. These costscan be significantly reduced with components which do not require lowtolerance alignments.

The prior patent (U.S. Pat. No. 6,234,634) advantageously teaches theuse of a wire grid polarizer as the PBS for both polarizing andanalyzing in an image projection system. However, the wire gridpolarizer itself presents various challenges. For example, the wire gridcan be fragile or susceptible to damage in environments with highhumidity, significant air pollution, or other conditions. Thus, it isdesirable to protect the wire grid. Because wire grid polarizers arewavelength sensitive optical devices, imbedding the polarizer in amaterial or medium with an index of refraction greater than one willalways change the performance of the polarizer over that available inair for the same structure. Typically, this change renders the polarizerless suitable for the intended application. Imbedding the polarizer,however, provides other optical advantages. For example, imbedding thepolarizer may provide other beneficial optical properties, and mayprotect the polarizer, although the performance of the polarizer itself,or polarization, may be detrimentally effected. Therefore, it isdesirable to obtain the optimum performance of such an imbeddedwire-grid polarizer.

Wire grids are typically disposed on an outer surface of a substrate,such as glass. Some wire grids have been totally encased in thesubstrate material, or glass. For example, U.S. Pat. No. 2,224,214,issued Dec. 10, 1940, to Brown, discloses forming a polarizer by meltinga powdered glass packed around wires, and then stretching the glass andwires. Similarly, U.S. Pat. No. 4,289,381, issued Sep. 15, 1981, toGarvin et al., discloses forming a polarizer by depositing a layer ofmetallization on a substrate to form the grid, and then depositingsubstrate material over the grid. In either case, the wires or grid aresurrounded by the same material as the substrate. As stated above, suchencasement of the wires or grids detrimentally effects the opticalperformance of the grid.

U.S. Pat. No. 5,748,368, issued May 5, 1998, to Tamada et al., disclosesa narrow bandwidth polarizer with a grid disposed on a substrate, and awedge glass plate disposed over the grid. A matching oil is also appliedover the elements which is matched to have the same refractive index asthe substrate. Thus, the grid is essentially encased in the substrate orglass because the matching oil has the same refractive index. Again,such encasement of the grid detrimentally effects the opticalperformance of the gird.

The key factor that determines the performance of a wire grid polarizeris the relationship between the center-to-center spacing, or period, ofthe parallel grid elements and the wavelength of the incident radiation.If the grid spacing or period is long compared to the wavelength, thegrid functions as a diffraction grating, rather than as a polarizer, anddiffracts both polarizations (not necessarily with equal efficiency)according to well-known principles. When the grid spacing or period ismuch shorter than the wavelength, the grid functions as a polarizer thatreflects electromagnetic radiation polarized parallel to the gridelements, and transmits radiation of the orthogonal polarization.

The transition region, where the grid period is in the range of roughlyone-half of the wavelength to twice the wavelength, is characterized byabrupt changes in the transmission and reflection characteristics of thegrid. In particular, an abrupt increase in reflectivity, andcorresponding decrease in transmission, for light polarized orthogonalto the grid elements will occur at one or more specific wavelengths atany given angle of incidence. These effects were first reported by Woodin 1902 (Philosophical Magazine, September 1902), and are often referredto as “Wood's Anomalies”. Subsequently, Rayleigh analyzed Wood's dataand had the insight that the anomalies occur at combinations ofwavelength and angle where a higher diffraction order emerges(Philosophical Magazine, vol. 14(79), pp. 60-65, July 1907). Rayleighdeveloped an equation to predict the location of the anomalies (whichare also commonly referred to in the literature as “RayleighResonances”).

The effect of the angular dependence is to shift the transmission regionto larger wavelengths as the angle increases. This is important when thepolarizer is intended for use as a polarizing beam splitter orpolarizing turning mirror because such uses require high angles ofincidence.

A wire grid polarizer is comprised of a multiplicity of parallelconductive electrodes supported by a substrate. Such a device ischaracterized by the pitch or period of the conductors; the width of theindividual conductors; and the thickness of the conductors. A beam oflight produced by a light source is incident on the polarizer at anangle Θ from normal, with the plane of incidence orthogonal to theconductive elements. The wire grid polarizer divides this beam into aspecularly reflected component, and a non-diffracted, transmittedcomponent. For wavelengths shorter than the longest resonancewavelength, there will also be at least one higher-order diffractedcomponent. Using the normal definitions for S and P polarization, thelight with S polarization has the polarization vector orthogonal to theplane of incidence, and thus parallel to the conductive elements.Conversely, light with P polarization has the polarization vectorparallel to the plane of incidence and thus orthogonal to the conductiveelements.

In general, a wire grid polarizer will reflect light with its electricfield vector parallel to the wires of the grid, and transmit light withits electric field vector perpendicular to the wires of the grid, butthe plane of incidence may or may not be perpendicular to the wires ofthe grid as discussed here.

Ideally, the wire grid polarizer will function as a perfect mirror forone polarization of light, such as the S polarized light, and will beperfectly transparent for the other polarization, such as the Ppolarized light. In practice, however, even the most reflective metalsused as mirrors absorb some fraction of the incident light and reflectonly 90% to 95%, and plain glass does not transmit 100% of the incidentlight due to surface reflections.

Applicants' prior patent (U.S. Pat. No. 6,122,103) shows transmissionand reflection of a wire grid polarizer with two resonances which onlyaffect significantly the polarizer characteristics for P polarization.For incident light polarized in the S direction, the reflectivity of thepolarizer approaches the ideal. The reflection efficiency for Spolarization is greater than 90% over the visible spectrum from 0.4 μmto 0.7 μm. Over this wavelength band, less than 2.5% of the S polarizedlight is transmitted, with the balance being absorbed. Except for thesmall transmitted component, the characteristics of the wire gridpolarizer for S polarization are very similar to those of a continuousaluminum mirror.

For P polarization, and high angle of incidence, the transmission andreflection efficiencies of the wire grid are affected by the resonanceeffect at wavelengths below about 0.5 μm. At wavelengths longer than 0.5μm, the wire grid structure acts as a lossy dielectric layer for Ppolarized light. The losses in this layer and the reflections from thesurfaces combine to limit the transmission for P polarized light.

Applicants' prior patent (U.S. Pat. No. 6,122,103) also shows thecalculated performance of a different type of prior-art wire girdpolarizer, as described by Tamada in U.S Pat. No. 5,748,368. As statedabove, an index matching fluid has been used between two substrates suchthat the grid is surrounded by a medium of constant refractive index.This wire grid structure exhibits a single resonance at a wavelengthabout 0.52 μm. There is a narrow wavelength region, from about 0.58 to0.62 μm, where the reflectivity for P polarization is very nearly zero.U.S Pat. No. 5,748,368 describes a wire grid polarizer that takesadvantage of this effect to implement a narrow bandwidth wire girdpolarizer with high extinction ratio. The examples given in the Tamadapatent specification used a grid period of 550 nm, and produced aresonance wavelength from 800 to 950 nm depending on the grid thickness,conductor width and shape, and the angle of incidence. The resonanceeffect that Tamada exploits is different from the resonance whoseposition is described above. While the two resonances may be coincident,they do not have to be. Tamada exploits this second resonance.Furthermore, there are thin film interference effects which may comeinto play. The bandwidth of the polarizer, where the reflectivity forthe orthogonal-polarized light is less than a few percent, is typically5% of the center wavelength. While this type of narrow band polarizermay have some applications, many visible-light systems, such as liquidcrystal displays, require polarizing optical elements with uniformcharacteristics over the visible-spectrum wavelengths from 400 nm to 700nm.

A necessary requirement for a wide band polarizer is that the longestwavelength resonance point must either be suppressed or shifted to awavelength shorter than the intended spectrum of use. The wavelength ofthe longest-wavelength resonance point can be reduced in three ways.First, the grid period can be reduced. However, reducing the grid periodincreases the difficulty of fabricating the grid structure, particularlysince the thickness of the grid elements must be maintained to ensureadequate reflectivity of the reflected polarization. Second, theincidence angle can be constrained to near-normal incidence. However,constraining the incidence angle would greatly reduce the utility of thepolarizer device, and preclude its use in applications such asprojection liquid crystal displays where a wide angular bandwidthcentered on 45 degrees is desired. Third, the refractive index of thesubstrate could be lowered. However, the only cost-effective substratesavailable for volume manufacture of a polarizer device are severalvarieties of thin sheet glass, such as Corning type 1737F or Schott typeAF45, all of which have a refractive index which varies between 1.5 and1.53 over the visible spectrum.

As stated above, the wire grid polarizer can include a multiplicity ofparallel conductive electrodes supported by a substrate. The substrateitself, however, can have certain optical consequences that can limitthe utility of a wire grid polarizer used in such an image displaydescribed above. For example, the substrate can cause aberrations ofastigmatism and coma if a non-collimated beam of light passes throughthe substrate tilted at an angle. One reason cube polarizing beamsplitters are sometimes used is because light enters such cubepolarizers with the optic axis normal to the cube surface, thusminimizing these aberrations.

Light striking the substrate at other than normal incidence can sufferfrom a lateral shift in position along the sloping direction of thesubstrate. Consequently, a diverging light cone striking the substratesuffers astigmatic aberration and coma causing the otherwise round areaof the beam to become elongated in one direction. This, combined withchromatic aberration (color separation) as polychromatic light dispersesthrough the tilted substrate, causes unacceptable distortion in highquality imaging optical systems. These aberrations occur regardless ofthe flatness of the substrate. Therefore, flat plate transmissive opticscannot be used in imaging applications unless the aberrations arecorrected or rendered negligible.

SUMMARY OF THE INVENTION

It has been recognized that it would be advantageous to develop an imageprojection system capable of providing bright images and good imagecontrast, and which is inexpensive. It also has been recognized that itwould be advantageous to develop an image projection system with apolarizing beam splitter that reduces aberrations of astigmatism andcoma, and/or that produces a transmitted or reflected beam with reducedgeometric distortions. It also has been recognized that it would beadvantageous to develop an image projection system with a polarizingbeam splitter which is protected against environmental degradation andother sources of damage while reducing detrimental effects of theprotection on the performance of the beam splitter.

It also has been recognized that it would be advantageous to develop animage projection system with a polarizing beam splitter capable ofutilizing divergent light (or having a smaller F/#), capable ofefficient use of light energy or with high conversion efficiency, andwhich is durable. It also has been recognized that it would beadvantageous to develop an image projection system with a polarizingbeam splitter having a high extinction ratio, uniform response over thevisible spectrum, good color fidelity, that is spathic, robust andcapable of resisting thermal gradients.

It also has been recognized that it would be advantageous to develop animage projection system with a polarizing beam splitter capable of beingpositioned at substantially any incidence angle so that significantdesign constraints are not imposed on the image projection system, butsubstantial design flexibility is permitted. It also has been recognizedthat it would be advantageous to develop an image projection system witha polarizing beam splitter which efficiently transmits p-polarized lightand reflects s-polarized light across all angles in the entire cone ofincident light. It also has been recognized that it would beadvantageous to develop an image projection system with a polarizingbeam splitter which is light-weight and compact. It also has beenrecognized that it would be advantageous to develop an image projectionsystem with a polarizing beam splitter which is easy to align.

The invention provides an image projection system with a polarizing beamsplitter which advantageously is a wire grid polarizer. The wire gridpolarizing beam splitter has a generally parallel arrangement of thin,elongated elements. The arrangement is configured, and the elements aresized, to interact with electromagnetic waves of the source light beamto generally transmit one polarization of light through the elements,and reflect the other polarization from the elements. Light having apolarization oriented perpendicular to a plane that includes at leastone of the elements and the direction of the incident light beam istransmitted, and defines a transmitted beam. The opposite polarization,or light having a polarization oriented parallel with the plane thatincludes at least one of the elements and the direction of the incidentlight beam, is reflected, and defines a reflected beam.

The system includes a light source for producing a visible light beam.The polarizing beam splitter is located proximal to the light source inthe light beam. The system also includes a reflective liquid crystalarray. The array may be located proximal to the polarizing beam splitterin either the reflected or transmitted beam. The array modulates thepolarization of the beam, and creates a modulated beam. The array isoriented to direct the modulated beam back to the beam splitter. Thearrangement of elements of the beam splitter interacts withelectromagnetic waves of the modulated beam to again generally transmitone polarization and reflect the other polarization. Thus, the reflectedportion of the modulated beam defines a second reflected beam, while thetransmitted portion defines a second transmitted beam. The array altersthe polarization of the beam to encode image information on themodulated beam. The beam splitter separates the modulated polarizationfrom the unmodulated beam, thus making the image visible on a screen.

A screen is disposed in either the second reflected or secondtransmitted beam. If the array is disposed in the reflected beam, thenthe screen is disposed in the second transmitted beam. If the array isdisposed in the transmitted beam, then the screen is disposed in thesecond reflected beam.

Unlike the bulky, heavy beam splitters of the prior art, the beamsplitter of the present invention is a generally planar sheet. The beamsplitter is also efficient, thus providing greater luminous efficacy ofthe system.

In accordance with one aspect of the present invention, the beamsplitter advantageously includes an embedded wire grid polarizer with anarray of parallel, elongated, spaced-apart elements sandwiched betweenfirst and second layers. The elements form a plurality of gaps betweenthe elements, and the gaps advantageously provide a refractive indexless than the refractive index of the first or second layer. Preferably,the gaps include air or have a vacuum.

In accordance with another aspect of the present invention, the elementsof the wire grid polarizer can be disposed on a substrate. Preferably,the substrate is very thin, or has a thickness less than approximately 5millimeters, to reduce astigmatism, coma, and/or chromatic aberrations.In addition, the wire grid polarizer and substrate preferably transmitsa transmitted beam with reduced geometric distortions, preferably lessthan approximately 3 standard wavelengths per inch.

In accordance with another aspect of the present invention, thesubstrate preferably has a surface with a flatness less thanapproximately 3 standard wavelengths deviation per inch to reducedistortions in the reflected beam.

In accordance with another aspect of the present invention, the beamsplitter is capable of being oriented with respect to the light beam andthe modulated beam at incidence angles between approximately 0 to 80degrees.

In accordance with another aspect of the present invention, the lightbeam has a useful divergent cone with a half angle between approximately10 and 25°. The beam splitter is used at a small F-number, preferablybetween approximately 1.2 and 2.5.

In accordance with another aspect of the present invention, the beamsplitter has a conversion efficiency of at least 50% defined by theproduct of the s-polarization reflected light and the p-polarizationtransmitted light (R_(s)T_(p)). In addition, the s-polarizationtransmitted light and the p-polarization reflected light are both lessthan 5%. Furthermore, the percentage of reflected light and thepercentage of the transmitted light of the modulated beam is greaterthan approximately 67%.

In accordance with another aspect of the present invention, the systemmay include a pre-polarizer disposed between the light source and thebeam splitter, and/or a post-polarizer disposed between the beamsplitter and the screen.

Additional features and advantages of the invention will be apparentfrom the detailed description which follows, taken in conjunction withthe accompanying drawings, which together illustrate, by way of example,features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a schematic view of the general operation of a preferredembodiment of an image projection system of the present invention usinga wire grid polarizing beam splitter of the present invention.

FIGS. 1b and 1 c are schematic views of the image projection system ofthe present invention in different configurations.

FIG. 2a is a graphical plot showing the relationship between wavelengthand transmittance for both S and P polarizations of a preferredembodiment of the wire grid polarizing beam splitter of the presentinvention.

FIG. 2b is a graphical plot showing the relationship between wavelengthand reflectance for both S and P polarizations of a preferred embodimentof the wire grid polarizing beam splitter of the present invention.

FIG. 2c is a graphical plot showing the relationship between wavelength,efficiency and transmission extinction of a preferred embodiment of thewire grid polarizing beam splitter of the present invention.

FIG. 3 is a graphical plot showing the performance of the preferredembodiment of the wire grid polarizing beam splitter of the presentinvention as a function of the incident angle.

FIG. 4a is a graphical plot showing the theoretical throughputperformance of an alternative embodiment of the wire grid polarizingbeam splitter of the present invention.

FIG. 4b is a graphical plot showing the theoretical extinctionperformance of an alternative embodiment of the wire grid polarizingbeam splitter of the present invention.

FIG. 4c is a graphical plot showing the theoretical extinctionperformance of an alternative embodiment of the wire grid polarizingbeam splitter of the present invention.

FIG. 5a is a schematic view of the general operation of an alternativeembodiment of an image projection system of the present invention.

FIGS. 5b and 5 c are schematic views of the image projection system ofthe present invention in different configurations.

FIG. 6 is a schematic view of the general operation of an alternativeembodiment of an image projection system of the present invention.

FIG. 7 is a perspective view of the wire grid polarizing beam splitterof the present invention.

FIG. 8 is a cross sectional side view of the wire grid polarizing beamsplitter of the present invention.

FIG. 9 is a cross-sectional view of an embedded wire grid polarizer ofthe present invention.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the exemplary embodimentsillustrated in the drawings, and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the invention is thereby intended. Any alterations andfurther modifications of the inventive features illustrated herein, andany additional applications of the principles of the invention asillustrated herein, which would occur to one skilled in the relevant artand having possession of this disclosure, are to be considered withinthe scope of the invention.

As illustrated in FIG. 1a, a display optical train of an imageprojection system of the present invention, indicated generally at 10,is shown. The image projection system 10 advantageously has a wire gridpolarizer as the beam splitter, indicated generally at 14. The wire gridpolarizing beam splitter 14 (WGP-PBS) efficiently reflects light of onepolarization from a source 20 to a reflective liquid crystal array 26,and then efficiently transmits reflected light of the oppositepolarization to a display screen 25.

For adequate optical efficiency, the WGP-PBS 14 must have highreflectivity (R_(s)) of the desired polarization from the light source20, and it must have high transmissivity (T_(p)) of the oppositepolarization from the liquid crystals array 26. The conversionefficiency is proportional to the product of these two, R_(s)T_(p), sodeficiency in one factor can be compensated to some extent byimprovement in the other.

Examples of wire grid polarizing beam splitters 14 of the presentinvention advantageously show the following characteristics whichdemonstrate the advantage of using a WGP-PBS 14 of the present inventionas both the polarizer and analyzer in display devices for the visibleportion of the spectrum. Theoretical calculations of furtherimprovements indicate that even better polarizing beam splitters will beavailable.

Referring to FIGS. 2a and 2 b, the measured transmissivity andreflectivity, respectively, for both S and P polarizations of a WGP-PBSare shown. In FIG. 2c, the efficiency of the WGP-PBS is shown as theproduct of the transmissivity and reflectivity. In addition, theextinction is also shown in FIG. 2c. In FIGS. 2a-2 c, the WGP-PBS isoriented to reflect the s-polarization and transmit the p-polarizationat incident angles of 30°, 45° and 60°. For an image projection system,such as a projector, the product of the reflected s-polarization andtransmitted p-polarization (R_(s)T_(p)) must be large if source light isto be efficiently placed on the screen. On the other hand, for theresolution needed to achieve high information density on the screen, itis important that the converse product (R_(p)T_(s)) be very small (i.e.the transmission of s-polarized light multiplied by the reflection ofppolarized light must be small). It is clear from the figures that thewire grid polarizing beam splitter of the present invention meets thesestandards over the entire spectrum without degradation by Rayleighresonance or other phenomena.

Another important feature is a wide acceptance angle. This must be largeif light gathering from the source, and hence the conversion efficiency,is maximized. Referring to FIG. 3, the performance of the wire gridpolarizing beam splitter of the present invention is shown for variousportions of the light cone centered around the optical axis which isinclined at 45°. In FIG. 3, the first referenced angle is the angle inthe plane of incidence while the second referenced angle is the angle inthe plane perpendicular to the plane of incidence. It is clear that theWGP-PBS of the present invention is able to accept cones of light(either diverging or converging) with half-angles between approximately10 and 25°.

Referring to FIGS. 4a-4 c, theoretical calculations for an alternativeembodiment of a wire grid polarizing beam splitter indicate thatsignificantly larger light cones and/or other enhancements will bepossible. FIGS. 4a and 4 b show the theoretical throughput andextinction, respectively, of a wire grid polarizing beam splitter with aperiod p reduced to 130 nm. In addition, the grid height or thickness is130 nm; the line-spacing ratio is 0.48; the substrate groove depth is 50nm; and the substrate is BK7 glass. It should be noted in FIG. 4a thatthe throughput is grouped much more closely than the throughput shown inFIG. 2c. Therefore, performance can be improved by reducing the periodp. It should be noted in FIG. 4b that the extinction is significantlyincreased in comparison to FIG. 2c.

FIG. 4c shows the theoretical extinction of another alternativeembodiment of the wire grid polarizing beam splitter with the period pfurther reduced. The wavelength is 420 nm and the incidence angle is30°. It should be noted that the extinction increases markedly as theperiod p is reduced.

As indicated above, an important consequence of the ability to acceptlarger light cones with a WGP-PBS that will work well at large angles isthat the PBS no longer restricts the optical design of the imagingsystem. Thus, conventional light sources can be used, with the advantageof their low cost, cooler operation, small size, and low weight. Thewide range of angles over which the WGP-PBS works well makes it possiblefor the designer to position the other optical elements in favorablepositions to improve the size and operation of the display. Referring toFIGs. 1b and 1 c, the design flexibility provided by the wide range ofangles of the PBS of the present invention is demonstrated. As shown inFIG. 1b, the light source 20 and array 26 may be positioned closertogether, with both having a relatively small incident angle withrespect to the PBS 14. Such a configuration is advantageous for acompact design of the components of the system 10. Alternatively, asshown in FIG. 1c, the light source 20 and array 26 may be positionedfarther apart, with both having a relatively large incident angle. Inshould be noted that in either case, the incidence angles vary greatlyfrom the 45 degree angle typically required by traditional beamsplitters.

Yet other features of wire grids provide advantages for display units.The conventional technology requires the use of a glass cube. This cubeimposes certain requirements and penalties on the system. Therequirements imposed include the need to deal with thermal loading ofthis large piece of glass, the need for high quality materials withoutstress birefringence, etc., which impose additional cost, and the extraweight and bulk of the cube itself. The WGP-PBS of the present inventionadvantageously is a divided or patterned thin film that does not occupymuch volume and does not weigh very much. It can even be integrated withor incorporated into other optical elements such as color filters, tofurther reduce part count, weight, and volume of the projection system.

The WGP-PBS of the present invention is also very robust. Modern lightsources generate very high thermal gradients in the polarizerimmediately after the light is switched on. At best, this can inducethermal and stress birefringence which causes cross talk betweenpolarizations. At worst, it can delaminate multilayer polarizers orcause the cemented interface in a cube beam splitter to separate. Whatis more, the long duration of exposure to intense light causes somematerials to change properties (typically yellowing fromphoto-oxidation). However, wire grid polarizing beam splitters are madeof chemically inert metal that is well attached to glass or othersubstrate materials. They have been shown to withstand high temperaturesas well as long periods of intense radiation from light sources.

The WGP-PBS of the present invention also is easy to align. It is asingle part that needs to be adjusted to direct the source beam onto theliquid crystal array. This is the same simple procedure that would beused for a flat mirror. There is another adjustment parameter, namely,the angular rotation about the normal to the WGP surface. Thisdetermines the orientation of polarization in the light beam. Thisadjustment is not critical because the WGP functions as its own analyzerand cannot be out of alignment in this sense. If there are otherpolarizing elements in the optical train, the WGP-PBS should be orientedwith respect to their polarization, but slight misalignment is notcritical because: according to Malus' law, angular variation makes verylittle difference in the intensity transmitted by polarizers if theirpolarization axes are close to being parallel (or perpendicular).

In order to be competitive with conventional polarizers, the productR_(s)T_(p) must be above approximately 50%. This represents a lowerestimate which would only be practical if the WGP-PBS was able to gathersignificantly more light from the light source than the conventionalpolarizing beam splitters. The estimate of 50% comes from an assumptionthat the best conventional beam splitter, a modern MacNeille cube beamsplitter, can deliver an f/# of about f/2.5 at best. An optical systemwhich was twice as fast, or capable of gathering twice as much light,would then have an f/# of 1/2 of this value, or about f/1.8, which iscertainly a reasonable f/# in optical image projection systems. A systemwhich is twice as fast, and therefore capable of gathering twice thelight from the source, would approximately compensate for the factor of2 decrease in the R_(s)T_(p) product over the conventional cube beamsplitter, resulting in an equivalent projection system performance. Infact, since a WGP-PBS can potentially be used down below f/1.2 (a factorof four increase) this seemingly low limit can still produce very brightimages. Of course, an R_(s)T_(p) product which is over this minimumvalue will provide even better performance.

Another important performance factor is contrast in the image, asdefined by the ratio of intensities of light to dark pixels. One of thesignificant advantages of the WGP-PBS is the improved contrast overcompound incident angles in comparison to the prior art cube beamsplitter such as a McNeille prism. The physics of the McNeille prismpolarizes light by taking advantage of the difference in reflectivity ofS vs. P polarization at certain angles. Because S and P polarization aredefined with respect to the plane of incidence, the effective S and Ppolarization for a particular ray in a cone of light rotates withrespect to the ray along the optical axis as various rays within thecone of light are considered. The consequence of this behavior is thewell-known compound angle problem in which the extinction of thepolarizer is significantly reduced for certain ranges of angles withinthe cone of light passing through the polarizing beam splitter,significantly reducing the average contrast over the cone.

The WGP-PBS, on the other hand, employs a different physical mechanismto accomplish the polarization of light which largely avoids thisproblem. This difference in behavior is due to the fact that thepolarization is caused by the wire grids in the beam splitter which havethe same orientation in space regardless of the plane of incidence forany particular ray in the cone of light. Therefore, even though theplane of incidence for any particular ray is the same when incident on aMcNeille prism or a WGP, the polarization effect is only dependent onthe plane of incidence in the case of the McNeille prism, meaning thecompound angle performance of the WGP is much improved over thatprovided by the cube beam splitter.

The fact that the function of the WGP-PBS is independent of the plane ofincidence means that the WGP-PBS can actually be used with the wires orelements oriented in any direction. The preferred embodiment of theinvention has the elements oriented parallel to the axis around whichthe polarizer is tilted so that the light strikes the WGP-PBS at anangle. This particular orientation is preferred because it causes thepolarization effects of the surface reflections from the substrate to beadditive to the polarization effects from the grid. It is possible,however, to produce a WGP-PBS which functions to reflect theP-polarization and transmit the S-polarization (which is exactlyopposite what has been generally described herein) over certain rangesof incident angles by rotating the grid elements so they areperpendicular to the tilt axis of the WGP-PBS. Similarly, the gridelements can be placed at an arbitrary angle to the tilt axis to obtaina WGP-PBS which functions to transmit and reflect light withpolarizations aligned with the projection of this arbitrary angle ontothe wavefront in the light beam. It is therefore clear that WGP-PBSwhich reflect the P-polarization and transmit the S-polarization, orwhich reflect and transmit light with polarization oriented at arbitraryangles are included within this invention.

The compound angle performance advantage of the WGP-PBS provides aninherently more uniform contrast over the entire light cone, and is oneof the reasons the WGP is suitable for very small f-numbers. But, ofcourse, it is not the only factor affecting the image contrast. Theimage contrast is governed to a large extent by low leakage of theundesired polarization, but in this case the product T_(s)R_(p) is notthe important parameter, because the image generating array which liesin sequence after the first encounter with the beam splitter but beforethe second also takes part in the production of the image contrast.Therefore, the final system contrast will depend on the light valveperformance as well as the polarizer extinction. However, lower boundson the required beam splitter performance can be determined with theassumption that the light valve performance is sufficient enough that itcan be assumed to have an essentially infinite contrast. In this case,the system contrast will depend entirely on the beam splitterperformance.

Referring to FIG. 1a, there are two different functions fulfilled by thebeam splitter 14. The first is the preparation of the polarized lightbefore it strikes the liquid crystal array 26 or other suitable imagegeneration device. The requirement here is that the light besufficiently well polarized that any variations in the polarization ofthe light beam created by the light valve can be adequately detected oranalyzed such that the final image will meet the desired level ofperformance. Similarly, the beam splitter 14 must have sufficientperformance to analyze light which is directed by the light valve backto the beam splitter so that the desired system contrast performance isachieved.

These lower bounds can be determined fairly easily. For reasons ofutility and image quality, it is doubtful that an image with a contrastof less than 10:1 (bright pixel to adjacent dark pixel) would have muchutility. Such a display would not be useful for dense text, for example.If a minimum display system contrast of 10:1 is assumed, then anincident beam of light is required which has at least 10 times the lightof the desired polarization state over that of the undesiredpolarization state. In terms of polarizer performance, this would bedescribed as having an extinction of 10:1 or of simply 10.

The second encounter with the beam splitter 14 which is going to analyzethe image, must be able to pass the light of the right polarizationstate, while eliminating most of the light of the undesired state.Again, assuming from above a light beam with an image encoded in thepolarization state, and that this light beam has the 10:1 ratio assumed,then a beam splitter is desired which preserves this 10:1 ratio to meetthe goal of a system contrast of 10:1. In other words, it is desired toreduce the light of the undesired polarization by a factor of 10 overthat of the right polarization. This again leads to a minimum extinctionperformance of 10:1 for the analysis function of the beam splitter.

Clearly, higher system contrast will occur if either or both of thepolarizer and analyzer functions of the beam splitter have a higherextinction performance. It is also clear that it is not required thatthe performance in both the analyzer function and the polarizer functionof the beam splitter be matched for a image projection system to performadequately. An upper bound on the polarizer and analyzer performance ofthe beam splitter is more difficult to determine, but it is clear thatextinctions in excess of approximately 20,000 are not needed in thisapplication. A good quality movie projection system as found in aquality theater does not typically have an image contrast over about1000, and it is doubtful that the human eye can reliably discriminatebetween an image with a contrast in the range of several thousand andone with a contrast over 10,000. Given a need to produce an image with acontrast of several thousand, and assuming that the light valves capableof this feat exist, an upper bound on the beam splitter extinction inthe range of 10,000-20,000 would be sufficient.

The above delineation of the minimum and maximum bounds on the wire gridbeam splitter is instructive, but as is clear from the demonstrated andtheoretical performance of a wire grid beam splitter as shown above,much better than this can be achieved. In accordance with thisinformation, the preferred embodiment has R_(s)T_(p)≧65%, and R_(p) orT_(s) or both are ≧67%, as shown in FIGS. 2a-2 c. The preferredembodiment would also employ the wire grid polarizing beam splitter inthe mode where the reflected beam is directed to the image generatingarray, with the array directing the light back to the beam splitter suchthat it passes through, or is transmitted through, the beam splitter.This preferred embodiment is shown in FIG. 1a.

Alternatively, as shown in the image display system 60 of FIG. 5a, thewire grid polarizing beam splitter 14 may efficiently transmit light ofone polarization from the source 20 to the reflective liquid crystalarray 26, and then efficiently reflect the reflected light of theopposite polarization to the display screen 25. The second embodiment ofthe image projection system 60 is similar to that of the preferredembodiment shown in FIG. 1a, with the exception that the beam splitter14 would be employed in a manner in which the source beam of light istransmitted or passed through the beam splitter 14 and directed at theimage generating array 26, then is reflected back to the beam splitter14 where it is reflected by the beam splitter and analyzed before beingdisplayed on the screen 25.

Again, referring to FIGS. 5b and 5 c, the design flexibility provided bythe wide range of angles of the PBS of the present invention isdemonstrated. As shown in FIG. 5b, the array 26 and screen 25 may bepositioned closer together, with both having a relatively small incidentangle with respect to the PBS 14. Alternatively, as shown in FIG. 5c,the array 26 and screen 25 may be positioned farther apart, with bothhaving a relatively large incident angle.

As shown in FIG. 6, a third embodiment of image projection system 80provides an alternative system design which may assist in achieving adesired level of system performance. This third embodiment would includeone or more additional transmissive or reflective polarizers which workin series with the wire grid polarizing beam splitter to increase theextinction of either or both of the polarizing and analyzing functionsto achieve the necessary system contrast performance. Another reason foradditional polarizers would be the implementation of a polarizationrecovery scheme to increase the system efficiency. A pre-polarizer 82 isdisposed in the source light beam between the light source 20 and theWGP-PBS 14. A post-polarizer or clean-up polarizer 84 is disposed in themodulated beam, or the beam reflected from the array 26, between thearray 26 and the screen 25, or between the WGP-PBS 14 and the screen 25.The third embodiment would still realize the advantages of the wire gridbeam splitter's larger light cone, durability, and the other advantagesdiscussed above.

As shown in the figures, the image display system may also utilize lightgathering optics 90 and projection optics 92.

Referring to FIGS. 7 and 8, the wire grid polarizing beam splitter 14 ofthe present invention is shown in greater detail. The polarizing beamsplitter is further discussed in greater detail in co-pending U.S.application Ser. No. 09/390,833, filed Sep. 7, 1999, entitled“Polarizing Beam Splitter”, which is herein incorporated by reference.

As described in the co-pending application, the polarizing beam splitter14 has a grid 30, or an array of parallel, conductive elements, disposedon a substrate 40. The source light beam 130 produced by the lightsource 20 is incident on the polarizing beam splitter 14 with theoptical axis at an angle Θ from normal, with the plane of incidencepreferably orthogonal to the conductive elements. An alternativeembodiment would place the plane of incidence at an angle Θ to the planeof conductive elements, with Θ approximately 45°. Still anotheralternative embodiment would place the plane of incidence parallel tothe conductive elements. The polarizing beam splitter 14 divides thisbeam 130 into a specularly reflected component 140, and a transmittedcomponent 150. Using the standard definitions for S and P polarization,the light with S polarization has the polarization vector orthogonal tothe plane of incidence, and thus parallel to the conductive elements.Conversely, light with P polarization has the polarization vectorparallel to the plane of incidence and thus orthogonal to the conductiveelements.

Ideally, the polarizing beam splitter 14 will function as a perfectmirror for the S polarized light, and will be perfectly transparent forthe P polarized light. In practice, however, even the most reflectivemetals used as mirrors absorb some fraction of the incident light, andthus the WGP will reflect only 90% to 95%, and plain glass does nottransmit 100% of the incident light due to surface reflections.

The key physical parameters of the wire grid beam splitter 14 which mustbe optimized as a group in order to achieve the level of performancerequired include: the period p of the wire grid 30, the height orthickness t of the grid elements 30, the width w of the grids elements30, and the slope of the grid elements sides. It will be noted inexamining FIG. 8 that the general cross-section of the grid elements 30is trapezoidal or rectangular in nature. This general shape is also anecessary feature of the polarizing beam splitter 14 of the preferredembodiment, but allowance is made for the natural small variations dueto manufacturing processes, such as the rounding of corners 50, andfillets 54, at the base of the grid elements 30.

It should also be noted that the period p of the wire grid 30 must beregular in order to achieve the specular reflection performance requiredto meet the imaging fidelity requirements of the beam splitter 14. Whileit is obviously better to have the grid 30 completely regular anduniform, some applications may have relaxed requirements in which thisis not as critical. However, it is believed that a variation in period pof less than 10% across a meaningful dimension in the image (such as thesize of a single character in a textual display, or a few pixels in animage) is required to achieve the necessary performance.

Similarly, reasonable variations across the beam splitter 14 in theother parameters described, such as the width w of the grid elements 30,the grid element height t, the slopes of the sides, or even the cornerrounding 50, and the fillets 54, are also possible without materiallyaffecting the display performance, especially if the beam splitter 14 isnot at an image plane in the optical system, as will often be the case.These variations may even be visible in the finished beam splitter 14 asfringes, variations in transmission efficiency, reflection efficiency,color uniformity, etc. and still provide a useful part for specificapplications in the projection imaging system.

The design goal which must be met by the optimization of theseparameters is to produce the best efficiency or throughput possible,while meeting the contrast requirements of the application. As statedabove, the minimum practical extinction required of the polarizing beamsplitter 14 is on the order of 10. It has been found that the minimumrequired throughput (R_(s)T_(p)) of the beam splitter 14 in order tohave a valuable product is approximately 50%, which means either or bothof R_(p) and T_(s) must be above about 67%. Of course, higherperformance in both the throughput and the extinction of the beamsplitter will be of value and provide a better product. In order tounderstand how these parameters affect the performance of the wire gridbeam splitter, it is necessary to examine the variation in performanceproduced by each parameter for an incident angle of 45°, and probablyother angles of interest.

The performance of the wire grid beam splitter 14 is a function of theperiod p. The period p of the wire grid elements 30 must fall underapproximately 0.21 μm to produce a beam splitter 14 which has reasonableperformance throughout the visible spectrum, though it would be obviousto those skilled in the art that a larger period beam splitter would beuseful in systems which are expected to display less than the fullvisible spectrum, such as just red, red and green, etc.

The performance of the wire grid beam splitter 14 is a function of theelement height or thickness t. The wire-grid height t must be betweenabout 0.04 and 0.5 μm in order to provide the required performance.

The performance of the wire grid beam splitter 14 is a function of thewidth to period ratio (w/p) of the elements 30. The width w of the gridelement 30 with respect to the period p must fall within the ranges ofapproximately 0.3 to 0.76 in order to provide the required performance.

The performance of the wire grid beam splitter 14 is a function of theslopes of the sides of the elements 30. The slopes of the sides of thegrid elements 30 preferably are greater than 68 degrees from horizontalin order to provide the required performance.

As indicated above, other factors can effect the performance and/ordurability of the WG-PBS. For example, WG-PBS may be subjected tostrenuous optical environments, such as high flux illumination and otherphysically harsh conditions, for long periods of time, which can effectthe durability of the WG-PBS. Thus, it is desirable to protect theWG-PBS. As stated above, however, embedding the polarizer in a materialor medium with an index of refraction greater than one will alwayschange the performance of the polarizer over that available in air inthe same structure. Therefore, it is desirable to protect the polarizer,while optimizing its performance.

As illustrated in FIG. 9, an embedded wire grid polarizer of the presentinvention is shown, indicated generally at 200. The polarizer 200includes a first optical medium, material, layer or substrate 201; asecond optical medium, material or layer 203; and a plurality ofintervening elongated elements 205 sandwiched between the first andsecond layers 201 and 203. As indicated above, although certainadvantages are obtained by encasing or imbedding the elements, thepolarization or performance of the elements is detrimentally effected.Thus, the polarizer 10 of the present invention is designed to optimizethe performance when imbedded, as discussed below.

The first and second layers 201 and 203 have respective first and secondsurfaces 202 and 204 which face one another and the elements 205. Thelayers 201 and 203, or material of the layers, also have respectivefirst and second refractive indices. The first and second opticalmediums 201 and 203 each have a thickness t_(L1) and t_(L2), and areconsidered to be thick in an optical sense. They may be, for example,sheets of glass or polymer, an optical quality oil or other fluid, orother similar optical materials. The thickness t_(L1) or t_(L2) may beanywhere from a few microns to essentially infinite in extent.Preferably, the thickness t_(L1) and t_(L2) of the layers 201 and 203 isgreater than 1 micron. The optical media 201 and 203 may be the samematerials, such as two sheets of glass, or they may be chosen to bedifferent materials, such as an oil for material 203 and glass formaterial 201. The elements 205 may be supported by the first layer orthe substrate 201.

The intervening array of elements 205 includes a plurality of parallel,elongated, spaced-apart, conductive elements 205. The elements 205 havefirst and second opposite surfaces 205 a and 205 b, with the firstsurfaces 205 a facing towards the first surface 202 or first layer 201,and second surfaces 205 b facing towards the second surface 204 orsecond layer 203. The first surfaces 205 a of the elements 205 maycontact and be coupled to the first surface 202 of the first layer 201,while the second surfaces 205 b may contact and be coupled to the secondsurface 204 of the second layer 203, as shown in FIG. 9. The array ofelements 205 is configured to interact with electromagnetic waves oflight in the visible spectrum to generally reflect most of the light ofa first polarization, and transmit most of the light of a secondpolarization.

The dimensions of the elements 205, and the dimensions of thearrangement of elements 205, are determined by the wavelength used, andare tailored for broad or full spectrum visible light. The elements 205are relatively long and thin. Preferably, each element 205 has a lengththat is generally larger than the wavelength of visible light. Thus, theelements 205 have a length of at least approximately 0.7 μm (micrometersor microns). The typical length, however, may be much larger. Inaddition, the elements 205 are located in generally parallel arrangementwith the spacing, pitch, or period P of the elements smaller than thewavelength of light. Thus, the pitch will be less than 0.4 μm(micrometers or microns).

The period of the elements 205, and the choices of the materials for theoptical mediums 201 and 203 are all made to obtain and enhance thedesired interactions with the light rays 209, 211 and 213. The ray oflight 209 is typically an unpolarized beam of light containing roughlyequal amounts of the two polarizations known in the field as Spolarization and P polarization. However, the ray of light 209 may bealtered in specific applications to be partially or mostly of eitherpolarization as well. The period P of the elements 205 is chosen suchthat the wire grid will specularly reflect most of the light of the Spolarized light 211, and transmit most of the P polarized light 213.

The optical materials also are chosen to aid in this process. Forexample, it is possible to choose optical material 201 to be equallytransmissive to both S and P polarizations, while choosing an opticalmaterial 203 that would absorb S polarized light or otherwise aid in thetransmission of P polarized light and the reflection of S polarizedlight. In the preferred embodiment, the optical material composing thelayers 201 and 203 is glass. Other materials are also suitable dependingon the particular application. For example, the second layer 203 can bea sheet of glass or plastic. In addition the second layer can be a layerof vacuum deposited film, or optical thin film, such as silicon dioxide,silicon nitride, magnesium fluoride, titanium oxide, etc. The secondlayer 203 also can be formed by chemically treating the surface of theelements and first layer to leave a thin film, such as hexamethyldisilazane. Such a layer can be one or several atomic monolayers that isless sensitive to the environmental conditions. Alternatively, thischemical treatment may be chosen to actively impede the physicalmechanisms which cause the damage to the wire grid structure in theharsh environment. The second layer also can include a plurality offilms of material mentioned herein.

The intervening elongated elements 205 are not very large. They willtypically be arranged in a regular, ordered array having a period P onthe order of 0.3 μm or less, with the width w_(R) of the ribs 205 andthe width w_(s) of the spaces or gaps 207 separating the elements on theorder of 0.15 μm or less. The width of the elements 205 and the spaces207 can be varied to achieve desired optical performance effects, whichwill be further described below. The height or thickness t_(R) of theseelements 205 will typically be between that required for the elements tobe optically opaque (approximately 40 nm in the case of aluminum) up toa height of perhaps 1 μm. The upper bound is fixed by considerations ofmanufacturing practicality as well as optical performance. In thepreferred embodiment, the elements 205 are typically composed ofmaterials such as aluminum or silver if the polarizer is to be usedacross the entire visible spectrum. However, if it is only required in aparticular case to provide a polarizer which performs in a portion ofthe spectrum, such as in red light, then other materials such as copperor gold could be used.

An important factor to obtaining the optimum performance of the imbeddedwire grid polarizer 200 is the material disposed within the spaces orgaps 207. The gaps 207, formed between the elements 205, advantageouslyprovide a refractive index less than the refractive index of at leastone of the layers 201 and 203, such as the first layer 201. Applicantshave found that, when the gaps 207 provide a lower refractive index, theperformance of the polarizer 200 is improved over wire grids totallyencapsulated in a material with a constant refractive index. In thepreferred embodiment, this material will be air or vacuum, but forreasons of practicality or performance in certain applications, othermaterials may be used.

It is desirable that this material have the lowest refractive index npossible while meeting other necessary design constraints such asmanufacturability. These other constraints may require that the materialfilling the spaces 207 between the elongated elements 205 be the samematerial as that composing either or both of the optical materials 201and 203. Or, the material filling the spaces 207 between the elongatedelements 205 may be chosen to be a material different from the opticalmaterials 201 and 203.

As mentioned, in the preferred embodiment, the material in the spaces207 will be air or vacuum. Other materials which may be used includewater (index 1.33), magnesium fluoride (index 1.38) or other commonoptical thin film materials which can be deposited using evaporation,sputtering, or various chemical vapor deposition processes, opticaloils, liquid hydrocarbons such as naptha, toluene, etc. or othermaterials with suitably low indices. The material in the gaps 207 alsocan include plastics, or fluorinated hydrocarbons (Teflon).

In addition, the substrate 201 (FIG. 9) or 40 (FIG. 7) of the WG-PBS canaffect the performance of the WG-PBS. As stated above, orienting thesubstrate of the WG-PBS at an angle with respect to the light can resultin aberrations of astigmatism and coma when non-collimated light passestherethrough. These aberrations occur regardless of the flatness of thesubstrate. Therefore, flat plate transmissive optics cannot be used inimaging applications unless the aberrations are corrected or renderednegligible.

This problem can be avoided in an imaging application if the beam oflight containing the image is reflected from the front surface of theplate rather than passed through it, because it is the transit throughthe tilted substrate that causes the optical aberrations, such as alateral shift in position along the sloping direction of the substrate.Such a configuration requires a flat substrate in order to avoiddistortions in the beam that result in distortions in the final image.Depending on the application, flatness less than approximately 3standard wavelengths deviation per inch is preferable; less thanapproximately 1 standard wavelength deviation per inch is morepreferable; and less than {fraction (1/10)}^(th) standard wavelengthsdeviation per inch is most preferable.

For the transmitted beam, it is desirable to reduce astigmatism andchromatic aberration. Thus, the substrate preferably is very thin, orhas a thickness less than approximately 5 millimeters.

Another important consideration for the image system of the presentinvention is to have a transmitted wave from the WG-PBS be sufficientlyfree of geometric distortions. For the transmitted beam, the distortionsare typically caused by deviations from parallel between the twosurfaces of the substrate. It is preferable that the transmitted beamhave a geometric distortion less than approximately 3 standardwavelengths deviation per inch; more preferably less than approximately½ standard wavelength deviation per inch; and most preferably less thanapproximately {fraction (1/10)}^(th) standard wavelength deviation perinch.

It is to be understood that the above-described arrangements are onlyillustrative of the application of the principles of the presentinvention. Numerous modifications and alternative arrangements may bedevised by those skilled in the art without departing from the spiritand scope of the present invention and the appended claims are intendedto cover such modifications and arrangements. Thus, while the presentinvention has been shown in the drawings and fully described above withparticularity and detail in connection with what is presently deemed tobe the most practical and preferred embodiment(s) of the invention, itwill be apparent to those of ordinary skill in the art that numerousmodifications, including, but not limited to, variations in size,materials, shape, form, function and manner of operation, assembly anduse may be made, without departing from the principles and concepts ofthe invention as set forth in the claims.

What is claimed is:
 1. An image projection system, comprising: a) alight source capable of producing a visible light beam; b) a polarizingbeam splitter, located near the light source in the light beam andoriented at an angle with respect to the light beam, the beam splittercomprising: 1) a first transparent substrate having a first surfacelocated in the light beam with the light beam striking the first surfaceat an angle, and having a refractive index; 2) a second layer, separatefrom the first transparent substrate, having a refractive index; and 3)a generally parallel arrangement of thin, elongated, spaced-apartelements disposed between the first transparent substrate and the secondlayer, and forming a plurality of gaps between the elements, the gapsproviding a refractive index less than the refractive index of the firsttransparent substrate or the second layer, the arrangement beingconfigured and the elements being sized to interact with electromagneticwaves of the source light beam to generally (i) transmit light throughthe elements which has a polarization oriented perpendicular to a planethat includes at least one of the elements and the direction of theincident light beam, defining a transmitted beam, and (ii) reflect lightfrom the elements which has a polarization oriented parallel with theplane that includes at least one of the elements and the direction ofthe incident light beam, defining a reflected beam; c) a reflectivearray located near the polarizing beam splitter in either the reflectedor transmitted beam, the array modulating the polarization of the beamby selectively altering the polarization of the beam to encode imageinformation thereon and creating a modulated beam, the array beingoriented to direct the modulated beam back towards the polarizing beamsplitter; d) the beam splitter further being located in the modulatedbeam and oriented at an angle with respect to the modulated beam, andthe arrangement of elements of the beam splitter interacting withelectromagnetic waves of the modulated beam to generally (i) transmitlight through the elements which has a polarization orientedperpendicular to the plane that includes at least one of the elementsand the direction of the incident light beam, defining a secondtransmitted beam, and (ii) reflect light from the elements which has apolarization parallel with the plane that includes at least one of theelements and the direction of the incident light beam, defining a secondreflected beam, to separate out the unaltered polarization from themodulated beam; e) a screen located in either the second reflected beamor the second transmitted beam for displaying the encoded imageinformation.
 2. A system in accordance with claim 1, wherein thetransparent substrate has a thickness less than approximately 5millimeters.
 3. A system in accordance with claim 1, wherein thetransparent substrate has a flatness less than approximately 3 standardwavelengths deviation per inch.
 4. A system in accordance with claim 1,wherein the first transmitted beam has a geometric distortion less thanapproximately 3 wavelengths deviation per inch.
 5. A system inaccordance with claim 1, wherein the gaps between the elements includeair.
 6. A system in accordance with claim 1, wherein the gaps betweenthe elements have a vacuum.
 7. A system in accordance with claim 1,wherein the gaps between the elements include a material different frommaterials of the first transparent substrate and the second layer.
 8. Asystem in accordance with claim 1, wherein the gaps include a materialthat is a same material as the second layer.
 9. A system in accordancewith claim 1, wherein the gaps include a material that is a samematerial as the first transparent substrate.
 10. A system in accordancewith claim 1, wherein the gaps between the elements include water.
 11. Asystem in accordance with claim 1, wherein the gaps between the elementsinclude magnesium fluoride.
 12. A system in accordance with claim 1,wherein the gaps between the elements include oil.
 13. A system inaccordance with claim 1, wherein the gaps between the elements includehydrocarbon compounds.
 14. A system in accordance with claim 1, whereinthe gaps between the elements include plastic.
 15. A system inaccordance with claim 1, wherein the gaps between the elements includefluorinated hydrocarbon.
 16. A system in accordance with claim 1,wherein the arrangement has a configuration and the elements have a sizewhich would normally create a resonance effect in combination with oneof the layer or the substrate within the visible spectrum; and whereinthe gaps with a lower refractive index than the refractive index of oneof the layer or the substrate causes a shift of the normally occurringresonance effect to a lower wavelength, thereby broadening a band ofvisible wavelengths in which no resonance effect occurs.
 17. A system inaccordance with claim 1, wherein the second layer includes a film.
 18. Asystem in accordance with claim 1, wherein the second layer includes aplurality of films.
 19. A system in accordance with claim 1, wherein thesecond layer includes a vacuum deposited film selected from the groupconsisting of: silicon dioxide, silicon nitride, magnesium fluoride, andtitanium oxide.
 20. A system in accordance with claim 1, wherein thesecond layer includes a sheet of glass.
 21. A system in accordance withclaim 1, wherein the second layer includes a sheet of plastic.
 22. Asystem in accordance with claim 1, wherein the second layer includes afilm of hexamethal disilazane.
 23. A system in accordance with claim 1,wherein the beam splitter is a generally planar sheet.
 24. A system inaccordance with claim 1, wherein the beam splitter is oriented withrespect to the light beam or the modulated beam at an incident anglebetween approximately 0 to 80 degrees.
 25. A system in accordance withclaim 1, wherein the beam splitter is oriented with respect to the lightbeam or the modulated beam at incidence angles greater than 47 degreesor less than 43 degrees.
 26. A system in accordance with claim 1,wherein the light beam has a useful divergent cone with a half anglebetween approximately 10 and 25°.
 27. A system in accordance with claim1, wherein the beam splitter is used at an F-number less thanapproximately f/2.5.
 28. A system in accordance with claim 1, whereinthe beam splitter has a throughput of at least 50% defined by theproduct of the fractional amount of p-polarization transmitted light andthe fractional amount of s-polarization reflected light; and wherein thes-polarization transmitted light and p-polarization reflected light areboth less than 5%.
 29. A system in accordance with claim 1, wherein thebeam splitter has a throughput of at least 50% defined by the product ofthe fractional amount of s-polarization transmitted light and thefractional amount of p-polarization reflected light; and wherein thep-polarization transmitted light and s-polarization reflected light areboth less than 5%.
 30. A system in accordance with claim 1, wherein thebeam splitter has a throughput for the light beam of at least 65%,defined by the product of the fractional amount of reflected light andthe fractional amount of transmitted light; and wherein the percent ofreflected light or the percent of transmitted light is greater thanapproximately 67%.
 31. A system in accordance with claim 1, furthercomprising a pre-polarizer disposed between the light source and thebeam splitter.
 32. A system in accordance with claim 1, furthercomprising a post-polarizer disposed between the beam splitter and thescreen.
 33. A system in accordance with claim 1, wherein the array isdisposed in the reflected beam; and wherein the screen is disposed inthe second transmitted beam.
 34. A system in accordance with claim 1,wherein the array is disposed in the transmitted beam; and wherein thescreen is disposed in the second reflected beam.
 35. A system inaccordance with claim 1, wherein a) the arrangement of elements has aperiod less than approximately 0.21 microns, b) the elements have athickness between approximately 0.04 to 0.5 microns, and c) the elementshave a width of between approximately 30 to 76% of the period.
 36. Asystem in accordance of claim 1, wherein the elements each have a crosssection with a base, a top opposite the base, and opposite left andright sides; and wherein the sides form an angle with respect to thebase greater than approximately 68 degrees.
 37. A method for projectingan image, the method comprising: a) producing a source light beam havinga wavelength in a range between approximately 0.4 to 0.7 microns using alight source; b) substantially separating polarizations of the sourcelight beam using a polarizing beam splitter disposed in the source lightbeam, the polarizing beam splitter including: 1) a first layer having arefractive index; 2) a second layer, separate from the first layer,having a refractive index; 3) a generally parallel arrangement of thin,elongated, spaced-apart elements, disposed between the first and secondlayers, configured and sized to interact with electromagnetic waves ofthe source light beam to generally (i) transmit light through theelements which has a polarization oriented perpendicular to a plane thatincludes at least one of the elements and the direction of the incidentlight beam, defining a transmitted beam, and (ii) reflect light from theelements which has a polarization orientation that lies in the planethat includes at least one of the elements and the direction of theincident light beam, defining a reflected beam; 4) a plurality of gaps,formed between the elements and the first and second layers, configuredto provide a refractive index less than the refractive index of thefirst or second layers; c) modulating either the transmitted orreflected beam and creating a modulated beam by selectively altering thepolarization of the beam using an array disposed in either thetransmitted or reflected beam; d) substantially separating thepolarizations of the modulated beam using the polarizing beam splitterdisposed in the modulated beam, the elements interacting withelectromagnetic waves of the modulated beam to generally (i) transmitlight through the elements which has a polarization orientedperpendicular to plane that includes at least one of the elements andthe direction of the incident light beam, defining a second transmittedbeam, and (ii) reflect light from the elements which has a polarizationorientation that lies in the plane that includes at least one of theelements and the direction of the incident light beam, defining a secondreflected beam; and e) displaying either the second transmitted beam orthe second reflected beam on a screen.
 38. An image display system forproducing a visible image, the system comprising: a) a light sourceconfigured to emit a source light beam having a wavelength in a rangebetween approximately 0.4 to 0.7 microns; b) a liquid crystal arraypositioned and oriented to receive and modulate at least a portion ofthe source light beam to create a modulated beam containing imageinformation; c) a screen positioned and oriented to receive and displayat least a portion of the modulated beam; and d) a polarizing beamsplitter positioned and oriented to receive both the source light beamand the modulated beam, the polarizing beam splitter including: 1) afirst layer having a refractive index; 2) a second layer, separate fromthe first layer, having a refractive index; 3) a generally parallelarrangement of thin, elongated, spaced-apart elements, disposed betweenthe first and second layers, configured and sized to interact withelectromagnetic waves of the source light beam to generally (i) transmitlight through the elements which has a polarization orientedperpendicular to a plane that includes at least one of the elements andthe direction of the incident light beam, defining a transmitted beam,and (ii) reflect light from the elements which has a polarizationorientation that lies in the plane that includes at least one of theelements and the direction of the incident light beam, defining areflected beam, and interacts with the electromagnetic waves of themodulated beam to generally (i) transmit light through the elementswhich has a polarization oriented perpendicular to a plane that includesat least one of the elements and the direction of the modulated lightbeam, defining a second transmitted beam, and (ii) reflect light fromthe elements which has a polarization orientation that lies in the planethat includes at least one of the elements and the direction of themodulated light beam, defining a second reflected beam; and 4) aplurality of gaps, formed between the elements and the first and secondlayers, configured to provide a refractive index less than therefractive index of the first or second layers.
 39. A system inaccordance with claim 38, wherein the first layer is a substrate havinga thickness less than approximately 5 millimeters.
 40. A system inaccordance with claim 38, wherein the first layer is a substrate havinga flatness less than approximately 3 standard wavelengths deviation perinch.
 41. A system in accordance with claim 38, wherein the either ofthe first or second transmitted beams has a geometric distortion lessthan approximately 3 wavelengths deviation per inch.
 42. A system inaccordance with claim 38, wherein the gaps between the elements includeair.
 43. A system in accordance with claim 38, wherein the gaps betweenthe elements have a vacuum.
 44. A system in accordance with claim 38,wherein the gaps between the elements include a material different frommaterials of the first transparent substrate and the second layer.
 45. Asystem in accordance with claim 38, wherein the gaps include a materialthat is a same material as the second layer.
 46. A system in accordancewith claim 38, wherein the gaps include a material that is a samematerial as the first transparent substrate.
 47. A system in accordancewith claim 38, wherein the gaps between the elements include water. 48.A system in accordance with claim 38, wherein the gaps between theelements include magnesium fluoride.
 49. A system in accordance withclaim 38, wherein the gaps between the elements include oil.
 50. Asystem in accordance with claim 38, wherein the gaps between theelements include hydrocarbon compounds.
 51. A system in accordance withclaim 38, wherein the gaps between the elements include plastic.
 52. Asystem in accordance with claim 38, wherein the gaps between theelements include fluorinated hydrocarbon.
 53. A system in accordancewith claim 38, wherein the arrangement has a configuration and theelements have a size which would normally create a resonance effect incombination with one of the layers within the visible spectrum; andwherein the gaps with a lower refractive index than the refractive indexof one of the layers causes a shift of the normally occurring resonanceeffect to a lower wavelength, thereby broadening a band of visiblewavelengths in which no resonance effect occurs.
 54. A system inaccordance with claim 38, wherein the second layer includes a film. 55.A system in accordance with claim 38, wherein the second layer includesa plurality of films.
 56. A system in accordance with claim 38, whereinthe second layer includes a vacuum deposited film selected from thegroup consisting of: silicon dioxide, silicon nitride, magnesiumfluoride, and titanium oxide.
 57. A system in accordance with claim 38,wherein the second layer includes a sheet of glass.
 58. A system inaccordance with claim 38, wherein the second layer includes a sheet ofplastic.
 59. A system in accordance with claim 38, wherein the secondlayer includes a film of hexamethal disilazane.
 60. A system inaccordance with claim 38, wherein a) the arrangement of elements has aperiod less than approximately 0.21 microns, b) the elements have athickness between approximately 0.04 to 0.5 microns, and c) the elementshave a width of between approximately 30 to 76% of the period.
 61. Asystem in accordance with claim 38, wherein the array is disposed in thereflected beam, and wherein the screen is disposed in the secondtransmitted beam.
 62. A system in accordance with claim 38, wherein thearray is disposed in the transmitted beam, and wherein the screen isdisposed in the second reflected beam.
 63. An image projection system,comprising: a) a light source producing a visible light beam; b) apolarizing beam splitter located near the light source in the light beamand oriented at an angle with respect to the light beam, the beamsplitter comprising: 1) a first transparent substrate having a firstsurface located in the light beam with the light beam striking the firstsurface at an angle; 2) the first transparent substrate having athickness less than approximately 5 millimeters; and 3) a generallyparallel arrangement of thin, elongated, spaced-apart elements, disposedon the first transparent substrate, the arrangement being configured andthe elements being sized to interact with electromagnetic waves of thesource light beam to generally (i) transmit light through the elementswhich has a polarization oriented perpendicular to a plane that includesat least one of the elements and the direction of the incident lightbeam, defining a transmitted beam, and (ii) reflect light from theelements which has a polarization oriented parallel with the plane thatincludes at least one of the elements and the direction of the incidentlight beam, defining a reflected beam; c) a reflective array locatednear the polarizing beam splitter in either the reflected or transmittedbeam, the array modulating the polarization of the beam by selectivelyaltering the polarization of the beam to encode image informationthereon and creating a modulated beam, the array being oriented todirect the modulated beam back towards the polarizing beam splitter; d)the beam splitter further being located in the modulated beam andoriented at an angle with respect to the modulated beam, and thearrangement of elements of the beam splitter interacting withelectromagnetic waves of the modulated beam to generally (i) transmitlight through the elements which has a polarization orientedperpendicular to the plane that includes at least one of the elementsand the direction of the incident light beam, defining a secondtransmitted beam, and (ii) reflect light from the elements which has apolarization parallel with the plane that includes at least one of theelements and the direction of the incident light beam, defining a secondreflected beam, to separate out the unaltered polarization from themodulated beam; and e) a screen located in either the second reflectedbeam or the second transmitted beam for displaying the encoded imageinformation.
 64. A system in accordance with claim 63, furthercomprising: a) second layer, separate from the first transparentsubstrate; and b) the arrangement of elements being disposed between thefirst transparent substrate and the second layer; and c) a plurality ofgaps formed between the elements; and d) the gaps providing a refractiveindex less than a refractive index of the first transparent substrate orthe second layer.
 65. A system in accordance with claim 63, wherein thefirst transparent substrate having a flatness less than approximately 3standard wavelengths deviation per inch.
 66. A system in accordance withclaim 63, wherein either the first or second transmitted beams have ageometric distortion less than approximately 3 wavelengths deviation perinch.
 67. An image projection system, comprising: a) a light sourceproducing a visible light beam; b) a polarizing beam splitter locatednear the light source in the light beam and oriented at an angle withrespect to the light beam, the beam splitter comprising: 1) a firsttransparent substrate having a first surface located in the light beamwith the light beam striking the first surface at an angle; 2) the firsttransparent substrate having a flatness less than approximately 3standard wavelengths deviation per inch; and 3) a generally parallelarrangement of thin, elongated, spaced-apart elements, disposed on thefirst transparent substrate, the arrangement being configured and theelements being sized to interact with electromagnetic waves of thesource light beam to generally (i) transmit light through the elementswhich has a polarization oriented perpendicular to a plane that includesat least one of the elements and the direction of the incident lightbeam, defining a transmitted beam, and (ii) reflect light from theelements which has a polarization oriented parallel with the plane thatincludes at least one of the elements and the direction of the incidentlight beam, defining a reflected beam; c) a reflective array locatednear the polarizing beam splitter in either the reflected or transmittedbeam, the array modulating the polarization of the beam by selectivelyaltering the polarization of the beam to encode image informationthereon and creating a modulated beam, the array being oriented todirect the modulated beam back towards the polarizing beam splitter; d)the beam splitter further being located in the modulated beam andoriented at an angle with respect to the modulated beam, and thearrangement of elements of the beam splitter interacting withelectromagnetic waves of the modulated beam to generally (i) transmitlight through the elements which has a polarization orientedperpendicular to the plane that includes at least one of the elementsand the direction of the incident light beam, defining a secondtransmitted beam, and (ii) reflect light from the elements which has apolarization parallel with the plane that includes at least one of theelements and the direction of the incident light beam, defining a secondreflected beam, to separate out the unaltered polarization from themodulated beam; and e) a screen located in either the second reflectedbeam or the second transmitted beam for displaying the encoded imageinformation.
 68. A system in accordance with claim 67, furthercomprising: a) a second layer, separate from the first transparentsubstrate; and b) the arrangement of elements being disposed between thefirst transparent substrate and the second layer; and c) a plurality ofgaps formed between the elements; and d) the gaps providing a refractiveindex less than a refractive index of the first transparent substrate orthe second layer.
 69. A system in accordance with claim 67, wherein thefirst transparent substrate has a thickness less than approximately 5millimeters.
 70. A system in accordance with claim 67, wherein eitherthe first or second transmitted beams have a geometric distortion lessthan approximately 3 wavelengths deviation per inch.
 71. An imageprojection system, comprising: a) a light source producing a visiblelight beam; b) a polarizing beam splitter located near the light sourcein the light beam and oriented at an angle with respect to the lightbeam, the beam splitter comprising: 1) a first transparent substratehaving a first surface located in the light beam with the light beamstriking the first surface at an angle; and 2) a generally parallelarrangement of thin, elongated, spaced-apart elements, disposed on thefirst transparent substrate, the arrangement being configured and theelements being sized to interact with electromagnetic waves of thesource light beam to generally (i) transmit light through the elementswhich has a polarization oriented perpendicular to a plane that includesat least one of the elements and the direction of the incident lightbeam, defining a transmitted beam, and (ii) reflect light from theelements which has a polarization oriented parallel with the plane thatincludes at least one of the elements and the direction of the incidentlight beam, defining a reflected beam; c) a reflective array locatednear the polarizing beam splitter in either the reflected or transmittedbeam, the array modulating the polarization of the beam by selectivelyaltering the polarization of the beam to encode image informationthereon and creating a modulated beam, the array being oriented todirect the modulated beam back towards the polarizing beam splitter; d)the beam splitter further being located in the modulated beam andoriented at an angle with respect to the modulated beam, and thearrangement of elements of the beam splitter interacting withelectromagnetic waves of the modulated beam to generally (i) transmitlight through the elements which has a polarization orientedperpendicular to the plane that includes at least one of the elementsand the direction of the incident light beam, defining a secondtransmitted beam, and (ii) reflect light from the elements which has apolarization parallel with the plane that includes at least one of theelements and the direction of the incident light beam, defining a secondreflected beam, to separate out the unaltered polarization from themodulated beam; and e) a screen located in either the second reflectedbeam or the second transmitted beam for displaying the encoded imageinformation; and f) the first and second transmitted beams having ageometric distortion less than approximately 3 wavelengths deviation perinch.
 72. A system in accordance with claim 71, further comprising: a) asecond layer, separate from the first transparent substrate; and b) thearrangement of elements being disposed between the first transparentsubstrate and the second layer; and c) a plurality of gaps formedbetween the elements; and d) the gaps providing a refractive index lessthan a refractive index of the first transparent substrate or the secondlayer.
 73. A system in accordance with claim 71, wherein the firsttransparent substrate has a thickness less than approximately 5millimeters.
 74. A system in accordance with claim 71, wherein the firsttransparent substrate having a flatness less than approximately 3standard wavelengths deviation per inch.